>There are more reasons to do multiple boilers today than ever before. This creates a system that will save the most fuel and provide some protection in the case of boiler failure. If one boiler fails, the second can still give off heat. The home may not be comfortable depending on the time of year, but it will be warmer than no heat at all. The biggest reason to consider multiple boilers is that our hot water demand has increased over the last decade. We now have large whirlpool 90-gallon tubs, hot tubs, multi-head shower stalls, full baths for each bedroom, clothes washers, dishwashers, and 14+ gpm faucets. We are concerned about cutting heating costs, but are not concerned about hot water costs. We install high-efficiency boilers and furnaces, but we may not make the wisest choice for water heaters. The indirect water heater has become more popular over the last decade. Most indirect water heaters are well insulated and have a coil which boiler water goes through to heat the domestic water in the tank. There is another style where there is a tank in a tank, and the boiler water flows between the two tanks. Both styles will supply plenty of efficient hot water in a very well-insulated tank. Since the tank is not connected to a chimney, it does not cool down as rapidly. The chimney will pull lots of heat from anything connected to it, which stays hot all the time. As the tank size increases, the boiler output must increase. See chart. As hot water demand increases and heat loss decreases, it creates a problem in matching a boiler to an indirect system. The larger boiler will heat the water well, but will short-cycle during the heating cycle. If the boiler short-cycles, the efficiency will be reduced.
The idea of possibly using two boilers instead of one makes the most sense with smaller heat losses and large hot water demands. One boiler will fire and heat the home, with the assistance of boiler number two if needed. This is done with controls, which are plentiful today. When there is a domestic hot water demand, both boilers will fire up and heat the tank rapidly. The choice between two boilers and one becomes a fuel-saving decision, rather than an upfront cost choice. Two boiler installations will be more expensive than one larger boiler.
Things to look at when making this choice are what the heat loss and domestic hot water demand are. If the heat loss is considerably less than the domestic hot water demand, it would be a good choice, provided you do not have a long recovery time, until hot water may be drawn again. Let me explain. You have a large hot tub. Other than that, there are no big demands. You only fill the large tub about 3-4 times a week. You have ample time to recover from the indirect water heater, whether you are in the hot tub or immediately after someone takes a shower. Handle this one of three ways.
1. - Install two indirectsâone for all the hot water demands except the hot tub and one for the hot tub. Install a large boiler or two smaller boilers staged with a good staging control.
2. - Install one bigger tank and a large boiler or two smaller boilers fired as needed.
3. - Install one indirect sized for the hot water demand, but not the hot tub. Install one larger or two smaller boilers and a good staging control. Handle the large tub demand from a separate water heater. Below you will see some drawings of boilers with and without an indirect.
The decision to install two boilers is not uncommon today with heating loads larger than 150,000 BTUs. Install two modulating/condensing (mod/con) or cast-iron boilers and stage the firing. One boiler will fire and heat the home. If the water temperature cannot be raised to the required level, the second boiler will fire up. The second boiler will fire up and shut down multiple times on a call for heat. When the second boiler is off, your fuel savings are the greatest. This creates a larger fuel savings than just one larger mod/con or a cast iron boiler.
Here is a decision to install two boilers and an indirect water heater. This choice was made due to a large hot water demand and a smaller heat demand. One boiler will fire for heat, and the second boiler will fire if needed. It will not fire for every heating cycle. When the indirect calls, both boilers will fire. One may drop out and fire back up again before the call for tank recovery ends.
The decision here is to increase the domestic hot water storage for larger hot water demand. Multiple boilers were chosen due to the large domestic hot water demand and smaller heat load. The tanks are piped in a reverse return, which balances the flow between the two tanks. The boilers will need to be staged and fired on and off as required. The boilers will all fire on a call for hot water and drop out as the temperature in the tank increases. 
Here, the decision was made to have two indirect hot water tanks. One for the hot water in the whole house and one for a large demand, such as a whirlpool tub, hot tub, or whatever the demand is. Again, multiple boilers were chosen due to a large domestic demand and a smaller heat demand. This is the most efficient way to handle a large demand, as recovery times can be longer for such high volumes. If the big whirlpool tub is used occasionally and not used again for 24 hours or so, we can use smaller boilers because they can take longer to heat the bigger tank. We can prioritize the smaller tank, but not the bigger tank, as it will take longer to recover before we can use it again. This will also enable us to recover more quickly on the smaller tank, should demand arise. The downside of this installation is that the large demand will need to be piped directly from the tank to the large demand device.
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